Topic 14b – Group 7, the Halogens
Revision Notes
1)
Trends in physical properties of the elements
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2)
At room temperature, Cl2 is a pale green gas, Br2 is a brown liquid, I2 is a blue-black
solid.
In Group 7, boiling point increases down the group because the molecules have more
electrons and, therefore, stronger van der Waal’s forces which require more energy
to overcome them
Electronegativity is the ability of an atom to attract the electrons in a covalent bond
Down Group 7, the shared electrons are further from the nucleus and more shielded.
There is a weaker attraction for the bonding electrons down the Group
Trends in chemical properties of the elements
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The Group 7 elements are oxidising agents which gain an electron when they react
The ability to oxidise decreases down Group 7
This can be shown by halogen displacement reactions where elements higher up the
group will displace elements further down the group
This can be done with chlorine dissolved in water or by bubbling chlorine gas through
NaBr(aq) or NaI(aq)
Cl2 + 2NaBr  2NaCl + Br2
Pale green solution
Yellow solution/orange in hexane
Cl2 + 2NaI  2NaCl + I2
Pale green solution
Brown solution/purple in hexane
Br2 + 2NaI  2NaBr + I2
Orange solution
Brown solution/purple in hexane
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These equations can also be written in ionic form
Cl2 + 2Br-  2Cl- + Br2
Cl2 + 2I-  2Cl- + I2
Br2 + 2I-  2Br- + I2
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3)
Down the group the outer electron is further from nucleus and more shielded so
gained less easily
Trends in properties of the halides
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The halides (chlorides, bromides and iodides) are reducing agents which lose an
electron when they react
The ability to act as a reducing agent increases down the Group
Down the group the outer electron is further from nucleus and more shielded so lost
more easily
This can be shown by the reactions of the solid halides with concentrated sulphuric
acid
Sodium halide
Observations
Products
Type of reaction
NaF
Steamy fumes
HF(g)
Acid-base (F- acting as base)
NaCl
Steamy fumes
HCl(g)
Acid-base (Cl- acting as base)
NaBr
Steamy fumes
Colourless gas
Brown fumes
HBr(g)
SO2
Br2
Acid-base (Br- acting as base)
Redox (reduction product of H2SO4)
Redox (oxidation product of Br-)
NaI
Steamy fumes
Colourless gas
Yellow solid
Smell of bad eggs
Black solid/purple
fumes
HI(g)
SO2
S
H2S
I2(s/g)
Acid-base (I- acting as base)
Redox (reduction product of H2SO4)
Redox (reduction product of H2SO4)
Redox (reduction product of H2SO4)
Redox (oxidation product of I-)
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4)
Iodide ions, I-, can reduce the S from oxidation state +6 in H2SO4 to +4 in SO2 then
to 0 in S and finally to -2 in H2S
Bromide ions, Br-, can reduce the S from oxidation state +6 in H2SO4 to +4 in SO2
Fluoride and chloride cannot reduce the S in H 2SO4 under these conditions
Reactions of Cl-, Br- and I- with Silver Ions and Ammonia Solution
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Chloride ions, bromide ions and iodide ions produce coloured precipitates with
acidified silver nitrate solution. Fluoride ions do not produce a precipitate with AgNO 3
Ag+(aq) + Cl-(aq)  AgCl(s)
White precipitate
Ag+(aq) + Br-(aq)  AgBr(s)
Cream precipitate
Ag+(aq) + I-(aq) 
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5)
AgI(s)
Yellow precipitate
AgCl(s) dissolves in dilute ammonia solution, NH 3(aq)
AgBr(s) dissolves in concentrated ammonia solution but not in dilute ammonia.
AgI(s) does not dissolve, even in concentrated ammonia solution
Redox Reactions of Group 7 Elements
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Chlorine undergoes a redox reaction with water. This reaction is used in water
purification to kill bacteria
Cl2(g) + H2O(l)  HCl(aq) + HOCl(aq)
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The benefits to health of water treatment outweigh the risks associated with handling
toxic chlorine gas
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Chlorine reacts with cold, dilute sodium hydroxide solution. This reaction is used to
make household bleach
Cl2(g) + 2NaOH(aq)  NaCl(aq) + NaClO(aq) +H2O(l)
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NaClO is sodium chlorate(I) which is the active ingredient in bleach
In both of these reactions the oxidation state of Cl changes from 0 in Cl 2 to –1 in Cland +1 in OCl-. Cl is simultaneously oxidised and reduced. This is an example of
disproportionation
Topic 14a – Group 2, the Alkaline Earth Metals
Revision Notes
1)
Trends in Physical Properties Down Group 2 (Be-Ba)
a)
Atomic radius
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b)
Increases down the group
Extra electron shell, outer electron further from nucleus and more shielded
Increased nuclear charge outweighed by greater shielding and distance
1st ionisation energy
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c)
Decreases down the group
Extra electron shell, outer electron further from nucleus and more shielded
Increased nuclear charge outweighed by greater shielding and distance
Melting & boiling points
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Decrease down the group
Metallic bonding gets weaker down the group. Ionic radius of metal ions increases
down group so charge density decreases. Weaker attraction between metal ions and
delocalised electrons
2)
Trends in Chemical Properties Down Group 2 (Be-Ba)
a)
Reactivity with Water
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Reactivity increases down the group
Extra electron shell, outer electrons further from nucleus and more shielded so lost
more easily. Increased nuclear charge outweighed by greater shielding and distance
Mg reacts slowly with cold water. Calcium reacts more vigorously
Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g)
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Mg reacts rapidly with steam
Mg(s) + H2O(g)  MgO(s) + H2(g)
b)
Solubility of Sulphates
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Solubility of group 2 sulphates decreases down the group i.e. BaSO4 is the least
soluble group 2 sulphate
BaCl2(aq) is used to test for sulphates. A white precipitate is produced
Ba2+(aq) + SO42-(aq)  BaSO4(s)
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Barium sulphate is used for X-ray imaging and CT scans. It is most often used in
imaging of the oesophagus. It is not readily absorbed by the body and absorbs Xrays more strongly than lighter atoms
c)
Solubility of hydroxides
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Solubility of group 2 hydroxides increases down the group. Mg(OH) 2 is sparingly
soluble i.e. only a small amount dissolves
Group 2 hydroxides are alkaline and can be used to neutralise acids
Calcium hydroxide, Ca(OH)2, can be used to reduce soil acidity in agriculture
Magnesium hydroxide, Mg(OH)2, is found in milk of magnesia. This is used to treat
indigestion by neutralising excess HCl in the stomach (the Mg(OH)2 acts as an
antacid)